This invention relates to a toner for the dry development in an electrographic art.
Broadly speaking, methods for developing electrostatic latent images can be classified into three categories: liquid development using developers obtained by dispersing various kinds of pigments and/or dyes in dielectric organic liquids; two-component dry development using developers obtained by mixing toner particles dispersing a colorant such as carbon black in natural or synthetic resins, with carrier particles such as iron powder or glass beads, i.e., a cascade method, a fur brush method, a magnetic brush method, an impression method, a powder cloud method and the like; and one-component dry development using developers obtained by mixing with fluid agents toners composed of a mixture of natural or synthetic resins used for the magnetic brush method, magnetic particles such as magnetite and pigments and/or dyes. In the sense, the present invention relates to electrophotographic developing toners of the type which is directed to the latter two-component or one-component dry development system.
Toners which are conventionally employed in the two-component development system are generally in the form of fine powder which is obtained by dispersing a charge controller and a dye or pigment serving as a colorant in a synthetic or natural resin, while the one-component development system generally uses toners in the form of fine powder which contains magnetic powder such as magnetite, a charge controller and a dye or pigment all dispersed in a synthetic or natural resin. In the two component system, these toners are practically applied, for example, by a magnetic brush method as follows: the toners are first triboelectrically charged and attached onto carrier particles such as iron powder and placed in a magnetic field to permit the particles to take a brush form. While, in the one-component system, the toners are allowed to be triboelectrically charged by the triboelectrification with and electrophotographic photosensitive medium or a developing container or by mutual triboelectrification therebetween and shaped into a brush form under the influence of a magnetic field in practical application. In order to reproduce a clear image with reduced fogging, it is essential that the toner particles be uniform in charge amount and size. Various kinds of pigments and dyes serving as a charge controller or colorant are known including, for example, carbon black (Colour Index Pigment Black 7), Nigrosine (C.I. Solvent Black 7), Aniline Blue (C.I. Acid Blue 20), Calonyl Blue (C.I. Axoic Blue 3), Chrome Yellow (C.I. Pigment Yellow 34), Ultramarine Blue (C.I. Pigment Blue 29), Du Pont Oil Red (C.I. Solvent Red 24), Quinoline Yellow (C.I. Acid Yellow 3), Methylene Blue Chloride (C.I. Basic Blue 9), Malachite Green Oxalate (C.I. Basic Green 4), lamp black (C.I. Pigment Black 6), Rose Bengal (C.I. Acid Fed 93), other aliphatic acid metal salts, metal-containing dyes and the mixtures thereof. However, these dyes and pigments are poor in miscibility with resins, so that it takes a long period of melting and mixing time to uniformly disperse the dyes and pigments in the resins. In most cases, the resulting toners are not uniform in amount of the static charge thereof. In addition, the toners suffer from mechanical impact force when agitated in a magnetic brush and will be, upon development, finer in particle size than those as expected. As a result, fogging will appear on an electrostatic image and toner filthing is liable to form an iron powder on the surface of the photosensitive medium, reducing the durability of the developer.
The known toners using resins such as polystyrene, styreneacryl copolymer, polyamide, epoxy resin and the like, produce the above-mentioned disadvantages after only several thousand cycles of copying. In order to avoid this, it will suffice to increase mechanical strengths, i.e., molecular weight, of the resin. However, the increase of the molecular weight of the resin involves a disadvantage that the fixation either by application of heat or by solvent is made difficult. In addition, the powdering in the toner producing step will become difficult.